Polymers and copolymers crosslinked with free radical initiators, organic peroxides and/or azo initiators, are known to have superior properties, particularly compared to polymers crosslinked by sulfur cure. These properties include high heat ageing resistance, low compression set, decreased staining of metal or coated metal sheet and easy production of colored products which have color stability during crosslinking and during long periods of use. These properties make use of peroxide cure of great practical importance. A possible drawback for cure of polymers with free radicals from organic peroxides and azo initiators has been that if air is not excluded from the surface of the material during cure, a tacky surface due to cure inhibition by oxygen in the air may result.
In order to avoid tacky surfaces on objects fabricated using such free radical crosslinking by organic peroxides and/or azo initiators, it has been conventional to exclude air from contact with the surface during cure to avoid the cure inhibition caused by the presence of oxygen. Measures to exclude oxygen add to the cost and complexity of the cure step and sometimes it is difficult, as in the cases of cure in steam autoclaves and in the interior of hoses, to assure the complete exhaustion of air and oxygen. In some cases the manufacturer would like to switch from sulfur to peroxide cure and use existing hot air oven curing chambers. Curing with conventional peroxide systems under these circumstances would not be viable as a tacky surface would result.
U.S. Pat. No. 6,747,099, which is hereby incorporated herein by reference in its entirety, disclosed compositions for providing a tack free surface upon curing.
In order to simplify and reduce the cost and complexity of the cure step, various methods have been suggested for preventing surface cure inhibition by oxygen during free radical crosslinking. These methods have, for various reasons, met with little or no success in actual practice. In particular, none have provided a tack free surface while providing the most desirable physical property of peroxide (azo) cure; superior compression set at 150° C. for 70 hours, compared to about 100° C., i.e., lower temperature performance for the prior art.
Thus, it is desirable to have methods for curing polymers and copolymers that can be performed in the full or partial presence of atmospheric oxygen. It also is desirable to have elastomeric compositions that can be molded and that do not stick to the mold.